Method for manufacturing disk member

ABSTRACT

“MEANS TO SOLVE” A burring area Wb is axially compressed from above and from below between an upper die  5  and a lower die  6  to undergo a plastic deformation, thereby forming a cylindrical boss  1 B. A pressing surface of the lower die  6  is formed with an annular recess  6 A, whereby the burring area Wb is less susceptible to move radially outward in the process of plastic deformation. Accordingly, a base (a bottom portion) of the burring area Wb fills the annular recess  6 A without leaving any space.  
     “EFFECT” A cylindrical boss  1 B having no hiatus in its axial end can be formed.

TECHNICAL FIELD

The present invention relates to a method of manufacturing a disk member, and more particularly, to a method of manufacturing a disk member which is preferred for use as a swash plate of a swash plate compressor, for example.

BACKGROUND ART

A method of manufacturing a disk member which is used for a swash plate of a swash plate compressor is known in the art (see patent literature 1, for example).

According to the patent literature 1, a disk member 1 is manufactured by manufacturing steps shown in FIG. 4. Specifically, a through-opening Wa is initially formed through the center of a plate-shaped raw material W in the form of a disk, as shown in FIG. 4 (a). A burring operation is then applied to the margin of the through-opening Wa to cause the margin to rise upward to its feet to shape a burring area Wb in the form of a truncated cone, as shown in FIG. 4 (b).

A punch 4 is then passed through the inside of the burring area Wb, as shown in FIG. 5, and the burring area Wb is strongly compressed from above and from below in the axial direction between an upper die 5 and a lower die 6, thus causing a plastic deformation of the burring area around the through-opening Wa to form a boss WB having a greater wall thickness than the original wall thickness t of the plate-shaped raw material W (see FIGS. 4 (c) and 5). This operation also forms a body WA extending radially outward and continuing from the boss WB. This step is referred to as a “swaging step” in the art. Thus, according to the prior art, the boss WB has a top surface WB′ which is swollen upwardly of the upper surface of the original plate-shaped raw material W and a bottom surface WB″ which is slightly raised relative to the lower surface of the original plate-shaped raw material W.

Subsequent to the swaging step, a pair of upper and lower press dies are used to shape the margin of the boss WB by a finishing operation, thus manufacturing the disk member 1 as a final product (see FIG. 4 (d)).

Patent literature 1: Japanese Laid-Open Patent Application No. 2002-239663.

DISCLOSURE OF THE INVENTION Issue to be Solved by the Invention

With the manufacturing method of the prior art as mentioned above, there is a disadvantage that when the swaging step illustrated in FIGS. 4 (c) and 5 is completed, a hiatus is likely to occur around the outer periphery of the top end of the boss WB, as indicated by X in FIG. 6.

An investigation by the inventor of the present Application revealed that such hiatus of the boss WB is caused by a mechanism as mentioned below. Specifically, in the prior art practice, the pressing surface (lower surface) of the upper die 5 is formed with an annular recess 5A which is to be filled by a top portion of the burring area Wb while the pressing surface (upper surface) of the lower die 6 is formed with an annular projection 6A which is to pound up a bottom portion of the burring area Wb.

The distal end (top end) of the burring area Wb is placed into the annular recess 5A of the upper die 5 while the base (bottom end) of the burring area Wb is supported by the annular projection 6A of the lower die 6, and under this condition, the upper die 5 is lowered to cause a plastic deformation of the burring area Wb between the annular recess 5A of the upper die 5 and the annular projection 6A as well as an inwardly adjacent annular recess 6B of the lower die 6 to fill therebetween.

It is to be noted that during this process, the burring area Wb is in the form of a truncated cone which is tapered upwardly and that the shaping process takes place in a manner such that the boss WB assumes a position subsequent to the swaging step which is raised upwardly of the upper surface of the original plate-shaped raw material W (FIG. 4 (d)). In other words, the boss is formed so as to project above the body in the same direction as the direction in which the burring area is caused to rise to its feet. Accordingly, during the plastic deformation as the burring area Wb is compressed from above and from below by the upper die 5 and the lower die 6, the burring area Wb moves more easily in the radially outward direction than into the annular recess 5A of the upper die 5 (see FIG. 5), and it is found that this explains for a failure of filling the annular recess 5A of the upper die 5 up to its corner 5A′ during the plastic deformation of the burring area Wb.

An issue that a hiatus as indicated by X in FIG. 6 is likely to occur around the outer periphery of the top end of the boss WB when manufactured according to the prior art manufacturing method due to the described cause has been pointed out in the art.

It is then an object of the present invention to provide a method of manufacturing a disk member which is free from a hiatus around a boss.

Means to Solve the Issue

Specifically, the present invention relates to a method of manufacturing a disk member comprising a piercing step of forming a through-opening centrally through a plate-shaped raw material, a burring step of causing the margin of through-opening of the plate-shaped raw material to rise to its feet toward one side thereof to form a substantially cylindrical burring area, and a forging step of causing the burring area to undergo a plastic deformation to form a cylindrical boss and a disk-shaped body which is located outwardly of the boss; in which at least one end of the cylindrical boss is formed to project beyond the end face of the body which is located on the opposite side from the direction in which the burring area is caused to rise to its feet.

EFFECTS OF THE INVENTION

With the manufacturing method mentioned above, one end of the cylindrical boss is formed so as to project in the opposite direction from the direction in which the burring area is caused to rise to its feet, and accordingly the burring area which undergoes a plastic deformation is less susceptible to a movement in the radially outward direction.

Accordingly, during the manufacture of a disk member, the occurrence of a hiatus of the boss can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

“FIG. 1” (a) is a view of a manufacturing step of a first embodiment according to the present invention; (b) is a view of a manufacturing step which follows (a); (c) is a view of a manufacturing step which follows (b); and (d) is a view of a manufacturing step which follows (c);

“FIG. 2” is a cross section of an upper die, a lower die and a plate-shaped raw material which are used in the manufacturing step shown in FIG. 1 (c);

“FIG. 3” is a cross section of a disk member which is completed as a final product by the manufacturing steps shown in FIG. 1;

“FIG. 4” (a) is a view of a manufacturing step used in the prior art; (b) is a view of a manufacturing step which follows (a); (c) is a view of a manufacturing step which follows (b); and (d) is a view of a manufacturing step which follows (c);

“FIG. 5” is a cross section of an upper die, a lower die and a plate-shaped raw material used in the step shown in FIG. 4 (c);

“FIG. 6” is a cross section, to an enlarged scale, of an essential part of a disk member 1 manufactured according to the prior art;

“FIG. 7” (a) is a view of a manufacturing step according to a second embodiment of the present invention; and (b) is a view of a manufacturing step which follows (a);

“FIG. 8” is a cross section, to an enlarged scale, of an essential part of a disk member manufactured by the manufacturing steps shown in FIG. 7;

“FIG. 9” (a) is a view of a manufacturing step according to a third embodiment of the present invention; and (b) is a view of a manufacturing step which follows (a);

“FIG. 10” is a cross section, to an enlarged scale, of an essential part of a disk member manufactured by the manufacturing steps shown in FIG. 9;

“FIG. 11” (a) is a view showing an manufacturing step according to a fourth embodiment of the present invention, and (b) is a view of a manufacturing step which follows (a);

“FIG. 12” is a cross section, to an enlarged scale, of an essential part of a disk member manufactured by the manufacturing steps shown in FIG. 11;

“FIG. 13” is a cross section of an essential part of a disk member manufactured by another embodiment of the present invention; and

“FIG. 14” is a cross section of an essential part of a disk member manufactured according to a further embodiment of the present invention.

DESCRIPTION OF CHARACTERS

-   -   1 . . . disk member     -   1A . . . body     -   1B . . . boss     -   1 a . . . lower surface of boss 1 B     -   1 b . . . lower surface of body 1A     -   5 . . . upper die (first die)     -   5A . . . annular projection     -   5B . . . annular recess     -   6 . . . lower die (second die)     -   6A . . . annular recess     -   W . . . plate-shaped raw material     -   Wa . . . through-opening     -   Wb . . . burring area

BEST MODES FOR CARRYING OUT THE INVENTION

Referring to the drawings, several embodiments of the present invention will now be described. FIG. 1 shows steps for manufacturing a disk member 1 in accordance with the present invention.

Before describing the manufacturing steps of the present embodiment with reference to FIG. 1, the disk member 1 as a final product manufactured according to the present embodiment will be described first. The disk member 1 of the present embodiment is intended to be used as a swash plate of a swash plate compressor. Specifically, as shown in FIG. 3, the disk member 1 comprises a centrally located, substantially cylindrical boss 1B having an increased wall thickness, and a disk-shaped body 1A continuing from the boss 1B and extending radially outward. The boss 1B has a wall thickness t1 (axial size) which is greater than the wall thickness t2 of the body 1A or the sheet thickness T of the original plate-shaped raw material W shown in FIG. 1. A lower surface 1 a which represents a lower end of the boss 1B projects downwardly beyond the lower surface 1 b of the body 1A. The boss 1B is centrally formed with a vertically extending through-opening 1F.

The boss 1B has a size difference between an external diameter and an internal diameter of the lower surface 1 a or a radial size t3, which is chosen substantially equal to the wall thickness t1 of the boss 1B. The lower surface 1 a of the boss 1B projects downwardly beyond the lower surface of the original plate-shaped raw material W or the lower surface 1 b of the body 1A.

A boundary 1C between the lower surface 1 b of the body 1A and the boss 1B is shaped to be arcuate in section. On the other hand, a boundary 1D between the top surface 1 d of the body 1A and the top surface 1 e of the boss 1B is shaped to be a shallow annular recess.

As mentioned previously, the disk member 1 is intended to be used as a swash plate of a swash plate compressor, and an attachment is fitted into the through-opening 1F in the boss 1B while either top surface 1 d or lower surface 1 b of the body 1A is disposed in sliding contact with a semi-spherical shoe.

The steps of manufacturing the disk member 1 according to the present embodiment will be described with reference to FIG. 1.

Initially, a plate-shaped raw material W which is cut into a circular form is provided, as shown in FIG. 1 (a). The plate-shaped raw material W has an external diameter D′ and a sheet thickness T which are chosen to be equal to the external diameter D of the body 1A of the disk member 1 as a final product shown in FIG. 3 and the wall thickness t2 of the body 1A. An inexpensive hot rolled steel sheet such as S45C, for example, is preferred for the plate-shaped raw material W.

A through-opening Wa having a required internal diameter is pierced centrally in the plate-shaped raw material W (FIG. 1 (a)). The through-opening Wa is circular in configuration, and has a center which coincides with the center of the plate-shaped raw material W. The piercing step which forms the through-opening Wa in the plate-shaped raw material W may take place by a laser cutting operation or a press stamping operation, but the press stamping operation is preferred for purpose of mass production.

When the piercing step of the plate-shaped raw material W is completed, the operation transfers then to a burring step shown in FIG. 1 (b). In the similar manner as in the conventional arrangement, this burring step takes place by using a die, not shown, having a guide opening of a greater diameter than the diameter of the through-opening Wa of the plate-shaped raw material W and disposed thereabove and an elevatable rod-shaped punch, not shown, which is disposed below the through-opening Wa in the plate-shaped raw material W. Thus, by driving the punch which is disposed below into and out of the guide opening in the die, the margin of the through-opening Wa is gradually folded upwardly of the plate-shaped raw material W toward the guide opening in the die.

When the burring step is completed, the entire circumferential area of the margin of the through-opening Wa is caused to rise to its feet upwardly, as shown in FIG. 1 (b), thus forming a sleeve-shaped burring area Wb in the form of a truncated cone which is tapered upwardly and a body 1A which is disposed radially outward thereof.

At the end of the burring step, the inner periphery Wb′ of the burring area Wb has an internal diameter which is on the order of twice the internal diameter of the original through-opening Wa. The inner periphery of the burring area Wb has a top end which is tapered, gradually decreasing the diameter. In the present embodiment, an arrangement is made so that at the end of the burring step, the internal diameter D1 at the top end of the inner periphery Wb′ coincides with the internal diameter of a through-opening 1F in the boss 1B of the disk member 1 as a final product.

It is to be noted that during the burring step, cracks in the folded portions can be prevented from occurring by merely applying a lubricant to the die or the punch or directly applying a lubricant to the margin of the through-opening Wa. Alternatively, the margin of the through-opening Wa may be softened as by annealing prior to the burring step.

When the burring step is completed, the operation transfers to a swaging (forging) operation.

The swaging (forging) step takes place by using an upper die 5 and a lower die 6 which have their pressing surfaces disposed in opposing relationship and a punch 4 which is passed through the burring area Wb to support it, as shown in FIG. 2. Specifically, the inner periphery Wb′ of the burring area Wb is supported by the punch 4, and under this condition, the lower die 6 is fixed at a given elevation while the upper die 5 is lowered by elevating means, not shown.

It is to be noted that the cross-sectional configurations of the pressing surfaces of the upper die 5 and the lower die 6 used in the present embodiment are formed in a manner opposite to the conventional arrangement. Specifically, the pressing surface (lower surface) of the upper die 5 is formed with an annular projection 5A which bulges toward the lower die 6 located therebelow, whereby an annular recess 5B is formed at a location adjacent to and located inwardly thereof. The annular projection 5A has a diameter which is slightly less than the external diameter of the top end of the burring area Wb. On the other hand, the pressing surface (top surface) of the lower die 6 is formed with an annular recess 6A. The annular recess 6A has an internal diameter which is slightly greater than the external diameter of bottom end portion Wb″ which represents the base of the burring area Wb′.

Accordingly, as the upper die 5 is lowered by elevating means, the annular projection 5A on the upper die 5 abuts against the top end of the burring area Wb while the plate-shaped raw material W is supported by the pressing surface (top surface) of the lower die 6, and further compresses the burring area Wb downward. At this time, the burring area Wb is supported from the inside by the punch 4.

In this manner, by compressing the burring area Wb in the axial direction from above and from below in a concentrated manner by the upper die 5 and the lower die 6 to cause a plastic deformation of the burring area Wb while maintaining the internal diameter of the burring area Wb around the inner periphery of the top end thereof, the boss 1B having an increased wall thickness is formed (FIG. 1 (c)).

Since the cross-sectional configurations of the pressing surfaces of the upper die 5 and the lower die 6 of the present embodiment are chosen to be opposite from those in a conventional arrangement, there is no displacement of the burring area Wb radially outward as the upper die 5 is lowered to cause a plastic deformation of the burring area Wb, allowing the annular recess 6A in the lower die 6 to be easily filled (FIG. 2). In addition, the top end of the burring area Wb is easily filled into the annular recess 5B in the upper die 5.

When the swaging step is completed in this manner, there are formed a cylindrical boss 1B having a lower surface 1 a and an upper surface 1 e which are annular flat surfaces disposed orthogonal to the axis and a body 1A which is disposed outwardly thereof, and the boss 1B is formed projecting beyond the lower surface of the plate-shaped raw material W (or the lower surface of the body 1A).

More specifically, the lower surface 1 a of the boss 1B which represents the bottom end is formed so as to project downwardly beyond the lower surface 1 b of the body 1A, which is disposed on the opposite side from the direction in which the burring area Wb is caused to rise to its feet, while the upper surface 1 e is formed so as to be recessed below the upper surface 1 d of the body 1A. A boundary region 1D which is formed by a shallow annular groove conforming to the cross-sectional configuration of the annular projection 5A of the upper die 5 is formed at a location adjacent to and outward of the upper surface 1 e of the boss 1B. In addition, an arcuate boundary region 1C which conforms to the cross-sectional configuration of the edge of the annular recess 6A of the lower die 6 is formed at a boundary between the lower surface 1 a of the boss 1B and the body 1A which is disposed outwardly thereof. Because the inner periphery of the boss 1B is supported by the punch 4, a through-opening 1F is shaped which has a uniform internal diameter over the entire axial extent thereof. Finally, the boss 1B has a wall thickness t1, which is greater than the wall thickness t2 of the body 1A.

When the swaging step shown in FIGS. 1 (c) and 2 has been completed, the plate-shaped raw material W is shaped into substantially the same configuration as the disk member 1 as a final product which is shown in FIG. 3. It is to be noted that during the swaging step, the outer peripheral surface and the upper and the lower surface of the body 1A may be supported by necessary support means which are disposed in a surrounding relationship so as to constrain the outer peripheral surface of the body 1A.

It is to be noted that the boss 1B may also be formed by utilizing the upper die 5, the lower die 6 and the burring area Wb which are disposed in an inverted manner from the illustration of FIG. 2. If the swaging operation takes place under this condition, it is possible to form the boss 1B having one axial end which projects in a direction opposite from the direction in which the burring area Wb is caused to rise to its feet.

When the swaging step has been completed, the operation then transfers to a finishing step. During the finishing step, press operations are applied to portions adjacent to and outward of the boss 1B in a concentrated manner using metal molds, not shown, and when the finishing step is completed, the boundary region 1C is shaped to be more strongly arcuate in section to form an annular groove having a shallow depth which surrounds the boss 1B, as shown in FIG. 1 (d), and thus the disk member 1 is shaped so as to be equivalent to a completed product shown in FIG. 3.

If the described swaging step is performed using the upper die 5 and the lower die 6 which are provided with finishing configurations, the finishing step can be omitted.

Subsequently, the lower surface 1 b and the upper surface 1 d of the body 1A of the disk member 1 which provide sliding surfaces with respect to a semi-spherical shoe or a piston are subject to a surface roughening, and the disk member 1 is chamfered at required locations and a surface coating is applied as required to complete the disk member 1 as the final product (FIG. 3).

As mentioned above, in the present embodiment, the upper die 5 and the lower die 6 which are used in the swaging operation shown in FIGS. 1 (c) and 2 are formed in the manner mentioned above, and the upper die 5 is lowered for operation. Accordingly, during a plastic deformation of the burring area Wb which occurs between the annular projection 5A of the upper die 5 and the annular recess 6A of the lower die 6, the burring area Wb is less susceptible to a movement in the radially outward direction, and accordingly, the burring area which undergoes a plastic deformation is completely filled into the annular recess 5B of the upper die 5 and the annular recess 6A of the lower die 6. The lower surface 1 a of the boss 1B which represents the bottom end is formed so as to project below the lower surface 1 b of the body 1A. Thus, the lower surface 1 a of the boss 1B which is located on the opposite side from the direction in which the burring area Wb is caused to rise to its feet is formed so as to project beyond the lower surface 1 b of the body 1A which is located outwardly thereof.

Accordingly, the occurrence of a hiatus in the outer periphery of the boss 1B as experienced in the prior art can be prevented, allowing a method of manufacturing a disk member 1 to be provided which is capable of preventing the occurrence of defective products as compared with the prior art.

FIG. 7 shows a second embodiment of the present invention. In the second embodiment, the cross-sectional configurations of the pressing surfaces of the upper die 5 and the lower die 6 used in the swaging step as shown in FIG. 7 (a) are changed, whereby the cross-sectional configurations of the boss 1B and associated parts are different from those of the first embodiment shown in FIG. 1 (c). Since the piercing step for the plate-shaped raw material W and the subsequent burring step remain the same as in the first embodiment, related Figures and corresponding description are omitted.

In the second embodiment, subsequent to the completion of the burring step, the upper die 5, the lower die 6 and the punch 4 which are similar to those shown in FIG. 2 are used to compress the burring area Wb in the axial direction to form the boss 1B. The lower surface 1 a of the boss 1B which represents the bottom end is formed so as to project below the body 1A, thus on the opposite side from the direction in which the burring area Wb is caused to rise to its feet (FIG. 7 (a)). On the other hand, the upper surface 1 e of the boss 1B is recessed below the upper surface 1 d of the body 1A. The swaging step also shapes the through-opening 1F. In this embodiment, the upper boundary region 1D is shaped so as to be gently tapered, and the lower boundary region 1C is also shaped to be tapered.

Subsequently, the finishing steps takes place in the similar manner as in the first embodiment to shape the junction between the boundary region 1C and a lower surface 1 b into an arcuate configuration (FIG. 7 (b)).

Finally, a chamferring, a surface roughening, and a coating are applied at required locations in the similar manner as in the first embodiment to complete the steps of manufacturing the disk member 1 as a final product (FIG. 8).

The second embodiment described above is capable of achieving a similar functioning and effects as in the first embodiment.

FIG. 9 shows a third embodiment of the present invention. In the third embodiment, by changing the cross-sectional configurations of the pressing surfaces of the upper die 5 and the lower die 6 shown in FIG. 9 (a) which illustrates the swaging step, the boss 1B and associated parts have different cross-sectional configurations from those in the first embodiment shown in FIG. 1 (c). The piercing step and the burring step which take place with respect to the plate-shaped raw material W remain unchanged from the first embodiment, and therefore will not be described.

In the third embodiment, subsequent to the completion of the burring step, using an upper die, not shown, having a pressing surface (lower surface) which is an entirely flat surface, a lower die 6 and a punch 4 which are similar to those shown in FIG. 2, the burring area Wb is compressed axially to form the boss 1B (FIG. 9 (a)). The boss 1B has an upper surface 1 e which is coplanar with an upper surface 1 d of the body 1A and a lower surface 1 a which projects downwardly of the lower surface 1 b of the body 1A on the opposite side from the direction in which the burring area Wb is caused to rise to its feet. Accordingly, a through-opening 1F is shaped during the swaging step, and a lower boundary region 1C is shaped to be tapered.

Subsequently, a finishing step takes place in the similar manner as in the first embodiment, shaping a junction between the boundary region 1C and a lower surface 1 b into an arcuate configuration (FIG. 9 (b)). At the same time, a boundary region 1D is formed which comprises an annular groove having a shallow depth and representing a boundary between the upper surface 1 e of the boss 1B and the upper surface 1 d of the body 1A. Finally, a chamferring, a surface roughening and a coating are applied at required locations in the similar manner as in the first embodiment, completing the steps of manufacturing the disk member 1 as a final product (FIG. 10).

Again, the third embodiment is capable of achieving a similar functioning and effects as in the first embodiment.

FIG. 11 shows a fourth embodiment of the present invention. In the fourth embodiment, by changing the cross-sectional configurations of pressing surfaces of an upper die 5 and a lower die 6 used in the swaging step shown in FIG. 11 (a), the boss 1B and associated parts have cross-sectional configurations which are different from those shown in the first embodiment illustrated in FIG. 1 (c). The piercing step and the burring step which take place with respect to the plate-shaped raw material W remain unchanged from the first embodiment, and therefore will not be described.

In the fourth embodiment, upon completion of the burring step, an upper die 5 which is similar to a conventional one shown in FIG. 5, and a lower die 6 and a punch 4 which are identical with those shown in FIG. 2 are used to compress the burring area Wb axially to form a boss 1B (FIG. 11 (a)).

When the swaging step is completed, the upper surface 1 e of the boss 1B is shaped to bulge above the upper surface 1 d of the body 1A while the lower surface 1 b of the boss 1B is shaped to project downwardly of the lower surface 1 b of the body 1A on the opposite side from the direction in which the burring area Wb is caused to rise to its feet. In addition, the swaging step shapes a through-opening 1F and shapes an upper boundary region 1D into an arcuate configuration as viewed in section, and also shapes a lower boundary region 1C to be arcuate in section.

Subsequently, a finishing step takes place in the similar manner as in the first embodiment, shaping both boundary regions 1C and 1D to be depressed into groove-shaped cross-sectional configurations (FIG. 11 (b)).

A chamferring, a surface roughening and a coating are then applied to required locations in the similar manner as in the first embodiment to complete the steps of manufacturing the disk member 1 as a final product (FIG. 12). Again, the fourth embodiment is capable of achieving a similar functioning and effects as in the first embodiment.

FIGS. 13 and 14 show disk members 1 having different cross-sectional configurations which are manufactured according to other embodiments of the present invention.

In the disk member 1 shown in FIG. 13, an upper boundary region 1D has a cross-sectional configuration which is arcuate in the similar manner as in the first embodiment while a lower boundary region 1C is shaped to be tapered. In other respects, the arrangement is identical with the disk member 1 of the first embodiment, and disk member 1 is manufactured by similar manufacturing steps as used in the first embodiment.

In the disk member 1 shown in FIG. 14, the both boundary regions 1C and 1D of the disk member 1 are arcuate in section. In other respects, the arrangement is similar to the disk member 1 of the first embodiment shown in FIG. 3, and the disk member 1 is manufactured by similar manufacturing steps as used in the first embodiment.

When manufacturing the disk members 1 shown in FIGS. 13 and 14, there are obtained a similar functioning and effects as in the first embodiment.

AVAILABILITY OF USE IN INDUSTRY

As described above, in accordance with the present invention, there is obtained an effect that the occurrence of a hiatus in a boss of a disk member can be prevented. 

1. A method of manufacturing a disk member comprising a piercing step of forming a through-opening centrally in a plate-shaped raw material, a burring step of causing the margin of the through-opening in the plate-shaped raw material to rise to its feet to one side of the plate-shaped raw material to form a substantially cylindrical burring area, and a forging step of causing the burring area to undergo a plastic deformation to form a cylindrical boss and a disk-shaped body which is located on the outside of the boss; in which at least one end of the cylindrical boss is caused to project beyond an end face of the body which is disposed on the opposite side from the direction in which the burring area is caused to rise to its feet.
 2. A method of manufacturing a disk member according to claim 1 in which the burring area is caused to undergo a plastic deformation by being compressed axially by a pressing surface of a first die and a pressing surface of a second die while the burring area is supported on the inside by a punch, thereby causing a base of the burring area to be filled into an annular recess formed in the pressing surface of the second die and thus forming the boss and the body.
 3. A method of manufacturing a disk member according to claim 2 in which the pressing surface of the first die is formed with an annular projection which bulges toward the second die and the first die is moved toward the second die to bring the annular projection on the first die into abutment against the distal end of the burring area, thereby forming the boss by axially compressing the burring area between the pressing surface of the first die and the pressing surface of the second die.
 4. A method of manufacturing a disk member according to claim 3 in which the boss is formed to have a wall thickness which is greater than the wall thickness of the body and the end of the boss which is disposed on the same side as the direction in which the burring area is caused to rise to its feet is formed so as to be recessed below the end face of the body which is located adjacent to and outward of the boss.
 5. A method of manufacturing a disk member according to claim 3 in which the boss is formed to have a wall thickness which is greater than the wall thickness of the body and the end of the boss which is disposed on the same side as the direction in which the burring area is caused to rise to its feet is formed to be coplanar with the end face of the body which is located adjacent to and outward of the boss.
 6. A method of manufacturing a disk member according to claim 3 in which the boss is formed to have a wall thickness which is greater than the wall thickness of the body and the end of the boss which is disposed on the same side as the direction in which the burring area is caused to rise to its feet is formed to project beyond the end face of the body which is located adjacent to and outward of the boss. 